Fast pyrolysis is a promising route to using biomass as a source of renewable energy and chemicals. For economic feasibility, this process has to be optimized with regard to product yield and handling. One of the big challenges in detailed process design is the complexity of biomass derived liquid mixtures, since they comprise hundreds of different organo-oxygenated chemicals, such as alcohols, ketones, aldehydes, furans, sugar derivatives, and also aromatic components if lignocellulosic biomass is processed. To model such a system, and to predict its phase behavior, an advanced thermodynamic model is required. In this work, we extend the group contribution associating equation of state (GCA-EOS) to lignin monomers and their Results show that GCA-EOS is able to handle this new family of organic compounds, not only their vapor-liquid equilibrium with other molecules typically found in the fast pyrolysis bio-oils, but also the liquid-liquid and solid-liquid equilibria.